1. Field of the Invention
This invention relates to electrical systems which contain positive temperature coefficient (PTC) circuit protection devices, particularly such systems in automobiles and other wheeled vehicles.
2. Introduction to the Invention
PTC devices are well known. Particularly useful devices contain PTC elements composed of a PTC conductive polymer, i.e. a composition comprising an organic polymer and, dispersed or otherwise distributed therein, a particulate conductive filler, e.g. carbon black, a metal or a conductive metal compound. Such devices are referred to herein as polymer PTC, or PPTC, devices. Other PTC materials are also known, e.g. doped ceramics, but are not as generally useful as PTC conductive polymer, in particular because they have higher resistivities. PTC devices can be used in a number of different ways, and are particularly useful in circuit protection applications, in which they function as remotely resettable fuses to protect electrical components from excessive currents and/or temperatures. Components which can be protected in this way include motors, batteries, loudspeakers and wiring harnesses in automobiles. The use of PPTC devices in this way has grown rapidly over recent years, and continues to increase. Reference may be made for example to U.S. Pat. Nos. 4,237,441, 4,238,812, 4,315,237, 4,317,027, 4,426,633, 4,545,926, 4,689,475, 4,724,417, 4,774,024, 4,780,598, 4,800,253, 4,845,838, 4,857,880, 4,859,836, 4,907,340, 4,924,074, 4,935,156, 4,967,176, 5,049,850, 5,089,801, 5,378,407, 5,451,919, 5,451,921, and 5,645,746, and to copending commonly assigned U.S. application Ser. No. 08/900,787 now U.S. Pat. No. 5,852,397, the disclosures of which are incorporated herein by reference for all purposes.
More recently, it has been proposed to combine PTC devices with other electrical components to provide circuit protection systems which respond to faults in ways which make use of the characteristics of both the PTC device and the electrical component. Reference may be made for example to U.S. Pat. Nos. 5,666,254 and 5,689,395, and to copending commonly assigned U.S. application Ser. Nos. 08/564,465 (Duffy et al., filed Nov. 29, 1995) now U.S. Pat. No. 5,864,458, 08/563,321 (Duffy et al., filed Nov. 28, 1995) now U.S. Pat. No. 5,745,322, 08/584,861 (Duffy et al., filed Jan. 5, 1996) abandoned in favor of continuation application Ser. No. 09/145,799, filed on Sep. 2, 1998,which has been abandoned in favor of continuation application Ser. No. 09/311,785 filed on May 14, 1999and now abandoned 08/658,782 (Duffy et al., filed Jun. 5, 1996) now abandoned 08/682,067 (Myong et al., filed Jul. 16, 1996) abandoned in favor of continuation application Ser. No. 09/156,933, filed Sep. 18, 1998 and 08/682,172 (Myong, filed Jul. 16, 1996) now abandoned and 08/868,905 (Myong, filed Jun. 4, 1997) abandoned in favor of continuation application Ser. No. 09/248,166 filed on Feb. 9, 1999 the disclosures of which are incorporated herein by reference for all purposes.
Ser. Nos. 08/682,067 and 08/682,172 describe protection systems which comprise a sensor element (typically a resistor) and a circuit interruption element (typically a relay) in series with the load, and a control element (typically a PTC device in parallel with the sensor element) which links the sensor element and the circuit interruption element so that an overcurrent is detected by the sensor element, and causes a change in the control element, which in turn causes a change in (typically opens) the circuit interruption element. Depending on the arrangement of these and other components, the system can (a) latch in an open state, either with or without a trickle current to keep the PTC device in a tripped condition, or (b) reclose if the overcurrent has gone away or cycle between open and closed positions if the overcurrent remains. Ser. No. 08/868,905 describes particularly useful devices for use in the protection systems of Ser. Nos. 08/682,067 and 08/682,172.
In the design of electrical systems for automobiles and like complex systems, it is usual for an electronic control module (ECM) to be connected to the electrical system through a connector bar having a number of separate contacts. The normal application of modular construction principles means that the number of contacts cannot easily be changed, and competition for space dictates that the number of contacts be kept to a minimum. It is, therefore, highly desirable for each electrical system connected to a connector bar to make use of a minimum number of contacts, preferably a single contact. For example, U.S. Pat. No. 5,645,746 refers to the possibility of monitoring the state of a PTC device by means of an LED or other voltage sensor; this works well, but requires the use of two positions on the connector bar.
In many electrical systems, it is desirable to incorporate diagnostics which will signal the condition of the system, at least when there has been a malfunction. Particularly is this so in automobiles and like complex systems in which the occurrence of a malfunction may not be immediately apparent. We have found that the protection systems described in Ser. Nos. 08/682,067 and 08/682,172 can be modified and combined with other components to provide both fault protection and diagnostic capabilities which do not depend upon manual interaction. For brevity and convenience, protection systems of the kind disclosed in Ser. Nos. 08/682,067 and 08/682,172 are referred to herein as TLPRR systems (TLPRR being an acronym for Thermally Linked PTC device, Resistor and Relay).
The first step in making use of TLPRR systems in diagnostics is to modify the TLPRR system disclosed in the earlier applications by replacing the momentary-on switch disclosed in those applications by a remotely-operated system which will activate the relay coil and thus close the relay contacts. Such a modified TLPRR system is referred to herein as a TLPRR (Mod) system. This modification can result in significant improvements even when not combined with diagnostic capabilities, and as such it forms an independent part of the present invention. Preferably the remotely-operated system comprises a resistor-capacitor (RC) network or other electrical system which occupies only a single position on a connector bar at a power distribution side. However, it is also possible to use a two-line system which activates the relay coil simply by passing a current through it for a sufficient time. Another important advantage of an RC circuit is that it can be designed to produce other useful functionality. The capacitor can be charged up by closing a switching unit at the return side of the coil (also known as low-side switching), and must have a size such that its discharge provides current to the relay coil long enough to close the relay contacts. Thereafter, the capacitor is charged up to the system voltage and acts as an open switch under normal operating conditions. If an overcurrent occurs, tripping the PTC device and opening the relay contacts, the sizes of the capacitor and the resistor will be significant factors in determining the time which will elapse before the TLPRR (Mod) attempts to reset. Thus the capacitor and resistor can be selected so as to result in a desired delay. Another important advantage of the RC network is that it protects the relay contacts if, when the system is turned on, there is a high current short circuit which sufficiently drops the system voltage to a level below the required relay activation voltage. In this case, the RC network will not have enough voltage to charge up the capacitor sufficiently to close the relay.
An embodiment of the present invention comprises an electrical protection system which can be connected between an electrical power supply and an electrical load to form an operating circuit, the operating circuit having an on state and an off state and comprising a current carrying line and a return line, and which when so connected protects the circuit from overcurrents, the system having a normal operating condition and a fault condition, and comprising:
a. a set of relay contacts which, when the system is so connected, is connected in series between the power supply and the load, and has:
i. a closed state which permits the flow of a normal current, INORMAL, when the system is in the normal operating condition, and
ii. an open state which permits the flow of at most a reduced current, substantially less than INORMAL, when the system is in the fault condition;
b. a resistive device which, when the system is so connected, is connected in series with the set of relay contacts and the load, and has
i. a normal state, when the current in the system does not exceed the normal current, INORMAL, by a predetermined current amount, and
ii. a fault state, when the current in the system exceeds the normal current, INORMAL, by the predetermined amount;
c. a control element which comprises a series combination of
i. a PTC device thermally coupled with the resistive device; and
ii. a relay coil coupled with the relay contacts;
xe2x80x83the series combination being connected across the power supply, between the current carrying line and the return line, with the PTC device connected to the current carrying line and the relay coil coupled to the return line;
the set of relay contacts changing from its closed state to its open state, thereby causing the system to change from its normal operating condition to its fault condition, when the resistance of the PTC device increases by a predetermined resistance amount in response to the resistive element changing from its normal state to its fault state; and
d. a relay coil current supply means which enables current to be supplied to the relay coil, so that if the relay contacts are open, the only way in which they can be closed is by supplying current to the relay coil through the relay coil current supply means.